Reduced or westward hotspot offset explained by dynamo action in atmospheres of ultrahot Jupiters

Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany

^★ Corresponding author: wicht@mps.mpg.de

Received: 7 January 2025
Accepted: 9 May 2025

Abstract

Hot Jupiters are tidally locked, Jupiter-sized planets in close proximity to their host star, exhibiting equilibrium temperatures exceeding 1000 K. Photometric observations often reveal that the hotspot - the hottest location in the atmosphere - has shifted with respect to the substellar point. While both eastward and westward offsets have been observed, hydrodynamic simulations typically predict an eastward offset due to advection by a characteristic eastward flow. In ultrahot Jupiters, where equilibrium temperatures surpass 2000 K, increased ionization has enhanced the electrical conductivity, leading to substantial Lorentz forces that can significantly influence the atmospheric dynamics. Here we present magnetohydrodynamic numerical simulations of atmospheres in ultrahot Jupiters that fully capture nonlinear electromagnetic induction effects. Our study identifies a novel magnetic instability that profoundly alters the dynamics, characterized by the disruption of the well-known laminar mean flows. This instability is triggered by a sufficiently strong background magnetic field with a realistic amplitude of around 1 G, assumed to originate from a deep-seated dynamo. Upon increasing the background field to 2.5 G, a subcritical dynamo mechanism emerges, capable of sustaining itself even when the external background field is removed. While hydrodynamic models exhibit a typical eastward offset, the magnetic instability results in either a vanishing or a westward hotspot displacement. Our results suggest that radial flow patterns associated with the instability play a significant role in modifying the hotspot position, providing a new mechanism to explain the diversity of observed hotspot shifts.